You don't need to raise & lower the nylon line, they'll bend well enough.
I've taken some light test fishing line (the only reason I've been in a
sporting goods store for years) and put a small bit into a tie, just taller
than axle height. One will hold a couple of cars nicely, although these
are not steep grades. They're also really hard to see, so they're
definitely not intrusive.
The main place I have them is at a freight house, so I have them set at the
right place for each door.
david d zuhn Saint Paul Bridge & Terminal Ry.
zoo @ stpaulterminal.org